virtio_net: fix a typo in virtnet_alloc_queues()

Obviously it should check !vi->rq.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>